Area Forecast Discussion
Issued by NWS Blacksburg, VA

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823 FXUS61 KRNK 250503 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 103 AM EDT Thu May 25 2017 .SYNOPSIS... A warm front across the region will lift north overnight ahead of a weak cold front that should cross the area by early Thursday. An area of upper level low pressure will then swing across the area Thursday before exiting Thursday evening. Weak high pressure spills in on Friday with a brief return to drier weather to end the week. && .NEAR TERM /THROUGH TODAY/... As of 1230 AM EDT Thursday... Flash flood watch has been allowed to expire as expecting rain to slowly taper and push east overnight. However will maintain higher pops for lingering shower bands with runoff of earlier heavy rainfall the main issue overnight. Previous discussion as of 745 PM EDT Wednesday... Deep synoptic scale long wave trof was over the eastern United states with two upper low within the trof. The trof crosses the Ohio Valley on Thursday with the main trof axis moving toward the East Coast on Tuesday afternoon. Have trimmed the Tornado Watch and expanded the Flash Flood Watch based on current radar trends and discussion with both SPC and WPC. The threat for flash flooding will continue until the cold front passes. Expecting rainfall rates typical in convective storms. Precipitable water values increase to around 1.5 inches in the piedmont of Virginia and North Carolina tonight. Rates of 1.0 to 1.5 inches per hour would exceed what is needed for flash flooding in parts of southwest Virginia and northwest North Carolina. The Flash Flood Watch will be expanded to include another tier of counties from Pulaski to Halifax. Cold pool aloft with the southern of the two upper lows passes over the Carolinas but 500MB temperatures still lower enough to produce large lapse rates over southeast West Virginia, southwest Virginia, and northwest North Carolina on Thursday. CAPES are forecast around 800 J/Kg in the piedmont Thursday afternoon. Low level moisture will erode east of the Blue Ridge Thursday but will increase with the increasing west wind on the west side of the central Appalachians. As the warm front moves north temperatures will temporarily go up then they will cool off again behind the cold front. Definitely a non-diurnal trend. Expecting enough breaks in the clouds east of the Blue Ridge on Thursday for some sun and temperatures warmer than Wednesday. Blend of MET/MAV guidance looks reasonable for maximum temperatures on Thursday. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... As of 245 PM EDT Wednesday... Thursday night into Friday, the center of the upper low responsible for the wet weather on Thursday will translate from near the Mason- Dixon line to off the coast of Cape Cod. This transition will place the region on the backside of the departing system within a tightening pressure gradient. Look for winds to become gusty from the northwest as 850mb flow is progged to peak around 40kts in the evening. Ridgetop gusts in the 30 to 40 mph range will be common with the highest peaks experiencing a few mph about that. Mountain valleys will experience gusts more in the 20 to 30 mph range. Gusts of any substantial nature are not expected downstream of the crest of the Blue Ridge. Precipitation will linger through the overnight hours in the form of upslope scattered showers. The best coverage will be across southeast West Virginia with decreasing potential heading eastward to roughly the Interstate-81 corridor. On Friday, we will gradually lose the upslope scattered showers, and associated cloud cover, as low level winds start to back more westerly through the day with the approach of an upper level shortwave ridge. The arrival of drier air will also yield a decreasing trend in coverage. The best coverage will still be over parts of southeast West Virginia, mainly during the morning hours. The vast majority of the forecast area will experience limited cloud cover and weak winds. Our period of drier weather will be short lived. Friday night into Saturday we will be watching a cold front approach and then enter the region. Isolated showers may on the radar screen by sunrise Saturday in the western portions of the area, but the better chances will be during the day Saturday with the arrival of the front combined with daytime heating. Guidance varies on what extent the front makes it through the region before stalling. Model are leaning towards a thunderstorm complex potentially riding along this stalled front late Saturday night and entering the area by Sunday morning. Temperatures during this portion of the forecast will start near normal, but trend to a little over five degrees above normal by Saturday. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 205 PM EDT Wednesday... During this part of the forecast, a broad, slow moving upper low will transition from the Great Lakes region to over southeast Canada. This will keep our region in a pattern dominated by a persistent trough of low pressure across the eastern portion of the U.S. Within this trough pattern a series of cold fronts are expected to enter, and then move through the forecast region. Guidance offers the first of these two to have a Sunday night into Monday time frame, with a second during late Tuesday night into Wednesday. Both of these time frames will be periods with a greater potential for showers and storms, with the first being more robust than the second. However, while the area is within the overall trough pattern, daytime heating will be enough to produce at least isolated showers during other parts of this time period. Temperatures during this portion of the forecast will start the period about ten degrees above normal. By Wednesday, readings within a degree or two of normal are forecast. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... As of 1245 AM EDT Thursday... Poor flying conditions to prevail overnight as a complex low pressure system and associated cold front swing through from the west. Expecting showers to slowly wane through the early morning hours as deeper moisture lifts northeast. However still expecting sub-VFR cigs overnight with MVFR to IFR vsbys in fog. High confidence in timing of the models bringing the cold front through the region after midnight. Downslope winds on the east side of the Blue Ridge will erode clouds and fog once mixing begins Thursday morning. Low level moisture and VFR stratocumulus will remain over the mountains. Scattered showers and isolated thunderstorms will develop in the afternoon. Some of the stronger thunderstorms may have small hail and MVFR visibilities along with heavy rain. Extended Aviation Discussion... Friday will be drier with a better chance of VFR ceilings and visibilities. Another front reaches the area for Saturday with more showers and thunderstorms including sub-VFR conditions into Monday. && .HYDROLOGY... As of 1255 AM EDT Thursday... Showers continue in bands across the region tonight leading to high creek and stream levels with minor flooding in spots. As rainfall rates slowly decrease expecting some improvement although may take until early Thursday for levels to start receding. Significant rises will also occur on mainstem rivers as well with both the New and Greenbrier likely reaching bankfull overnight. Minor Flooding is expected to continue on the Dan River at South Boston and is forecast on the Dan River at Paces and on the Roanoke River at Randolph. Another round of showers and storms will occur again Thursday afternoon which given low flash flood guidance values could lead to added flooding issues. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS/JH NEAR TERM...JH/AMS SHORT TERM...DS LONG TERM...DS AVIATION...AMS/JH HYDROLOGY...AMS

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